In recent years, the nanotechnology is being developed quickly. In particular, CNT is receiving attention owing to the excellent characteristics thereof. Specifically, CNT is excellent in electric characteristics, mechanical strength and the like, and is greatly expected as a filler constituting a composite material with a resin, an organic semiconductor or the like, with unlimited future applications to electronic devices, electrochemistry and the like. CNT has a size (diameter) of several nanometers and thus is increasingly expected as a material, such as a probe and the like, as transporting means for a medical component, such as an anticancer agent, an antiviral agent and the like, into the living body in the medical and pharmaceutical fields, and as a compositional component for cosmetics. Fullerenes, such as C60, C70 and the like, carbon nanohorns and carbon nanocapsules are in the same situations. The CNT, fullerenes and carbon nanocapsules referred herein encompasses, in addition to pure carbon clusters, carbon clusters that partly have heteroatoms substituted or heteroatoms intercalated.
However, CNT, carbon nanohorns, fullerenes and the like are difficulty soluble in a hydrophilic solvent, such as water, an organic solvent (e.g., an alcohol, acetic acid and the like) and the like, in spite of the excellent characteristics thereof. Among CNT, single wall carbon nanotubes (SWCNT) exhibit higher insolubility than multi-wall carbon nanotubes (MWCNT). Single wall carbon nanohorns (SWCNH) are in the same situations. Owing to the nature thereof, the carbon nanomaterial is prevented from being spread practically in contrast with the great expectation thereof.
A hydrophilic solvent, such as water and the like, easily dissolves a polar solute having high hydrophilicity owing to the high hydrophilicity thereof, but the carbon nanomaterial, such as CNT, fullerenes and the like, is difficulty dissolved therein due to the non-polarity (hydrophobicity) thereof. Accordingly, such methods have been practiced as a chemical bonding method of subjecting CNT or fullerenes to surface modification by effecting a chemical treatment, such as an acid treatment and the like, to forma carboxyl group on the surface, and a physical adsorption method of physically adsorbing a solubilizing agent, such as a surfactant and the like, on the surface of CNT or fullerenes for solubilization (for example, Patent Documents 1 and 2). In the physical adsorption method, ultrasonic vibration or the like is often applied for accelerating solubilization, after addition of the solubilizing agent. The physical adsorption method has such characteristics in that CNT suffers less structural defects formed on the surface thereof, in contrast with the chemical bonding method. The hydrophilic solvent referred herein means a solvent having a hydrophilic group and having a high dielectric constant.
Apart from the solubilizing treatment, the inventors have proposed an underwater high voltage pulsed arc discharge method as an effective production method of carbon nanoparticles (Non-patent Document 1). The inventors have found in the studies that carbon nanoparticles produced by the production method of Non-patent Document 1 are formed in a state where the carbon nanoparticles are uniformly dispersed in water.
In the production method reported in Non-patent Document 1, which is a revolutionary method that attains simultaneously both production and solubilization of carbon nanoparticles by performing pulsed arc discharge in water, however, the dispersion herein occurs incidentally upon producing the carbon nanotubes, and thus the method is not a universal solubilizing method capable of dissolving a carbon material produced by an arbitrary production method in a hydrophilic solvent. Furthermore, the high voltage pulsed arc discharge method utilized in herein is basically adapted for a treatment in a gas state since thermal plasma is used, and thus an apparatus therefor is not simple due to high energy necessarily used.
The term “solubilization” referred in the specification means that hydrophilic nature is applied to a hydrophobic solute that is difficultly soluble in a hydrophilic solvent, and the solute is dispersed in the solvent owing to the nature. Accordingly, the term “solubilization” referred in the specification does not mean a state without turbidity corresponding to emulsification, but is equivalent to a combination of a solubilizing treatment and a dispersing treatment (which generally forms turbidity) of an insoluble solute.
Such a dispersion method has been proposed in that multi-wall CNT is subjected to a plasma treatment in a gas state by utilizing low temperature plasma (nonequilibrium plasma) of oxygen, nitrogen or the like, which is different from the use of thermal plasma such as pulsed arc discharge in Non-patent Document 1, so as to obtain an acidic functional group content of 2% or more per carbon on the surface thereof, and thus is dispersed in a liquid with ultrasonic wave or the like (for example, Patent Document 3). The equipment for discharging in the method is simple owing to the use of low temperature plasma, as compared to the high voltage pulsed arc discharge method.
Patent Document 3 discloses such an assumption in that multi-wall CNT can be dispersed since the multi-wall CNT has an acidic functional group on the surface thereof, and the acidic functional group repulses an acidic functional group on another multi-wall CNT adjacent thereto to ravel out the tangled multi-wall CNT, which is thus dispersed. However, the operation, in which multi-wall CNT is subjected to a plasma treatment in a specific gas atmosphere to obtain an acidic functional group content of 2% or more, and then further subjected to a physical dispersing treatment in a liquid, such as ultrasonic wave, high-speed agitation and the like, involves increase in number of process steps due to the treatments in gas and liquid, which brings about complex treatments, prolonged process time, and use of bloated equipments, whereby complication in control and management induces increase in cost.
The conventional plasma treatment for surface modification of CNT utilizes discharge in a gas atmosphere, but discharge occurs not only in a gas atmosphere but also in water. This has been reported by the inventors in the production method of carbon nanoparticles in Non-patent Document 1.
It has been reported that radicals, such as an OH radical, an H radical, an O radical, an H2O2 radical and the like, and ozone O3 are generated upon performing pulsed streamer discharge in water (for example, Non-patent Documents 2 and 3). It has also been reported that the discharge plasma radiates a strong ultraviolet ray corresponding to about 30% of the energy of the plasma to activate the area along the channel of discharge, and H2O2 generated is decomposed to an OH radical with the ultraviolet ray (Non-patent Document 2). However, the pulsed streamer discharge reported in Non-patent Documents 2 and 3 is for generating radicals in water to process microorganisms or harmful chemical substances in water with the action of the radicals thus activated, but has no relationship to the problem of changing the insolubility, which is one of the physical natures of a carbon nanomaterial in a hydrophilic solvent, to a soluble nature, and resolution means therefor (i.e., a method of solubilizing a carbon nanomaterial for dissolving in a hydrophilic solvent). Examples of the case utilizing underwater streamer discharge for cleaning water as similar to the above include a wastewater processing equipment (for example, Patent Document 4).
It has been reported that in the case where gas is bubbled upon performing pulsed streamer discharge in water, a chemical action is directly applied in addition to the physical action, so as to generate radicals (Non-patent Document 4). According to Non-patent Document 4, in the case where the gas bubbled is oxygen, a certain amount of an OH radical is generated, and in the case of argon, the amounts of an H radical and an O radical are large, but the amount of an OH radical is small. However, Non-patent Document 4 does not suggest a solubilizing method of dissolving a carbon nanomaterial in a hydrophilic solvent, as similar to Non-patent Documents 2 and 3. Consequently, it is still unknown what type of contribution is made by streamer discharge in a solvent to solubilization of a carbon nanomaterial.    Patent Document 1: JP-A-8-12310    Patent Document 2: JP-A-2001-104771    Patent Document 3: JP-A-2003-300715    Patent Document 4: JP-A-2001-252665    Non-patent Document 1: J. Suehiro, K. Imasaka, Y. Ohshiro, G. Zhou, M. Hara, N. Sano, “Production of carbon nanoparticles using pulsed arc discharge triggered by dielectric breakdown in water”, Japan Journal Applied Physics, vol. 42, pp. 1483-1485 (2003)    Non-patent Document 2: B. Sun, M. Sato, J. S. Clements, “Non-uniform pulse discharge-induced radical production in distilled water”, Journal Electronics, vol. 43, pp. 115-126 (1998)    Non-patent Document 3: H. Akiyama, “Streamer discharge in liquid and their applications”, IEEE Trans. Electr. Insl., vol. 39, pp. 646-652 (2000)    Non-patent Document 4: B. Sun, M. Sato, J. S. Clements, “Optical study of active species produced by a pulsed streamer corona discharge in water”, Journal Electronics, vol. 39, pp. 189-202 (1997)